> I half wonder if adding a larger heatsink, or even putting thermal mass around the existing oscillator could also help, or if the heating is more localized in the PCB itself.

That would likely make it worse. The trick here is that the other cores are running at essentially their maximum temperature and and will dynamically reduce their clockspeed if required to keep from going above that limit. In essence, the environment becomes actively temperature controlled. If the ambient heat goes higher, the cores clock lower, if it gets colder the cores clock higher (up to a point).

If you add too much heat dissipation, the total power used by those cores might not be enough to keep well above ambient.

Extreme power dissipation would keep temperature stable so that this whole setup might not be needed, though.

Author should experiment with liquid nitrogen ;) [1]

[11] https://www.xda-developers.com/liquid-nitrogen-cooling-raspb...